1. Field of the Invention
This invention relates to the gasification of coal and other carbonaceous solids and related processes and is particularly concerned with a method for the removal of toxic trace element pollutants from aqueous effluents produced during coal gasification and similar operations.
2. Description of the Prior Art
One of the problems associated with the gasification of coal and similar carbonaceous solids is that of preventing the discharge of toxic trace elements into the environment. Studies have shown that most coals contain small amounts of cadmium, cobalt, lead, zinc, mercury, antimony, arsenic, and other elements which are toxic in low concentrations and could become hazardous pollutants. Some of these elements are retained for the most part as insoluble compounds in the ash formed during gasification and combustion operations but others, such as mercury, are volatile enough to be present in trace quantities in the product and flue gas streams produced during such operations. These streams are normally cooled for the recovery of heat and the removal of condensed steam and then scrubbed with aqueous solvents and water washed to remove carbon dioxide, hydrogen sulfide, hydrogen cyanide and similar acidic constituents. As a result of these gas cleanup operations, the volatile trace element constituents may be transferred into process water streams where they may tend to accumulate. Because of the low concentrations in which these materials are normally present in the gases and the limited use of coal gasification and related processes in recent years, there has been relatively little attention directed to the elimination of these materials from the aqueous effluents. It can be shown, however, that large gasification plants and similar installations may produce such materials in quantities sufficient to create serious problems if they are not removed from the effluents.